1. Field of the Invention
The present invention relates to an ink jet printhead, and, more particularly, to a printhead having an ink repellent coating adjacent the nozzles thereof to avoid the formation of puddles at the nozzles and ensure the directionality of the ink droplets ejected therefrom.
2. Description of Related Art
In ink jet printing, a printhead is usually provided having one or more ink-filled channels communicating with an ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. These printheads form images on a recording medium, such as paper, by expelling or ejecting droplets of ink from the nozzles onto the recording medium. The ink forms a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus. An important property of a high quality printhead is precise jet directionality. This ensures that ink droplets will be placed precisely where desired on the printed document. Poor jet directional accuracy results in the generation of deformed characters and visually objectionable banding in half tone pictorial images.
A major source of ink jet misdirection is associated with improper wetting of the front face of the printhead which contains the array of nozzles. One factor which adversely affects jet directional accuracy is the interaction of ink accumulated on the front face of the printhead with the ejected droplets. Ink may accumulate on the printhead face either from overflow during the refill surge of ink or from the spatter of small satellite droplets resulting from the process of expelling droplets from the printhead. When accumulated ink on the front face of the printhead makes contact with ink in the channel (and in particular with the ink meniscus at the nozzle orifice), the ink distorts which results in an imbalance of forces acting on the egressing droplet and in turn leads to jet misdirection. This wetting phenomenon becomes more troublesome after extensive use of the printhead as the face oxidizes or becomes covered with a dried ink film. As a result, a gradual deterioration of the generated image quality occurs. Thus, in order to retain good ink jet directionality, wetting of the front face of the printhead is preferably suppressed.
In thermal ink jet printing, a thermal energy generator, usually a solid state resistor, is located in the channels near the nozzles at a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels the ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. The rapidly expanding vapor bubble pushes the column of ink filling the channel towards the nozzle. At the end of the current pulse, the heater rapidly cools, and the vapor bubble begins to collapse. However, because of inertia, most of the column of ink that received an impulse from the exploding bubble continues its forward motion and is ejected from the nozzle as an ink drop. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move toward the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction toward a recording medium, such as paper. However, puddling of ink in contact with the nozzle orifice in the face of a thermal ink-jet printhead will cause deflection of the droplet and thus misdirection. Therefore, the wetting characteristics of the front face of the printhead are critical to accurate printing.
Ink jet printheads include an array of nozzles and may be formed of silicon wafers using orientation dependent etching (ODE) techniques. The use of silicon wafers is advantageous because ODE techniques can form structures, such as nozzles, on the wafers in a highly precise manner. Moreover, these structures can be fabricated efficiently at low cost. The resulting nozzles are generally triangular in cross-section. Thermal ink jet printheads made by the abovementioned ODE techniques are typically comprised of a channel plate which contains a plurality of nozzle-defining channels located on a lower surface thereof bonded to a heater plate having a plurality of resistive heater elements formed on an upper surface thereof and arranged so that a heater element is located in each channel. The upper surface of the heater plate may include an insulative layer which is patterned to form recesses exposing the individual heating elements. This insulative layer is referred to as a "pit layer" and is sandwiched between the channel plate and heater plate so that the nozzle-containing front face may have three layers; the channel plate, the pit layer and the heater plate.
The heater and channel plates are typically formed from silicon, while the pit layer, sandwiched between the heater and channel plates, is formed from a polymer, typically polyimide. Since the front face of the printhead includes these different materials, a coating material, such as water-repellent material, will not adhere equally well to these different materials, resulting in a coating which is not uniformly ink repellent. Thus, it is difficult to provide a surface coating which is uniformly ink repellent for ink jet printheads formed from multiple layers.
Additionally, these printers typically use an ink which contains a glycol and water. Glycols and other similar materials are referred to as humectants, which are substances which promote the retention of moisture. For a coating material to be effective for any length of time, it must both repel and be resistant to glycol-containing inks.
Further, it is difficult to apply a coating to the face of an ink jet nozzle. Many materials will not adhere sufficiently to the silicon wafer face. While it is desirable to suppress the wetting property of the nozzle jet surface, it may be undesirable to allow any coating material to enter the channel of the nozzle. If the walls of the channel become coated with ink-repellent material, proper refill of the channel may be inhibited. Refill of each channel depends on surface tension and must be completed in time for the subsequent volley of drops to be fired. If the refill process is not complete by the time the next drop is fired, the meniscus may not be flush with the outer edge of the nozzle orifice, resulting in misdirection. Further, an incompletely filled channel causes the ink drop size to vary, which also leads to print quality degradation.
In ink jet printers, which are not thermally controlled, conductive ink is directly heated by passing electrical energy through the channel of ink between two electrodes. For example, U.S. Pat. No. 4,595,937 to Conta et al. discloses an ink jet printhead having a ceramic base with a conductive plate on the backside and a conductive plate on the front side which serve as an electrode and counter electrode, respectively. A nozzle is drilled through the ceramic layer between the conductive plates, and corrosion resistant layer covers the outer face of the printhead.
Similarly, U.S. Pat. No. 4,703,332 to Crotti et al. also utilizes an electrode and counter electrode located on the front face of the printhead. However, these ink jet printheads are distinct from thermal ink jet printheads which do not normally include a conductive layer on the outer face. For example, U.S. Pat. No. 4,751,532 to Fujimura et al. discloses a thermal electrostatic ink jet printhead having an insulating layer on its outer face. The insulating layer is critical to maintaining the shape of the meniscus at the nozzle orifice since the shape of the meniscus greatly influences printing quality. The insulating layer is made of a silicone type of fluorocarbon-type resin having a low surface energy treatment. However, such a low surface energy treatment requires a very complicated process as described in Fujimura et al.
Thus, the ability to change the wetting characteristics of the front face of the printhead to simply and effectively ensure directionality of an ink droplet is needed.